1. Technical Field
This invention relates to the technology of preventing over spray of cavities of components and more particularly to masking complex components containing such cavities when carrying out thermal spraying within such cavities.
2. Discussion of the Prior Art
Thermal spraying of metal powder, droplets and other comminuted particles onto interior cylinder bore surfaces of an automotive engine block has become a significant, fabrication technique that forms a wear and anti-friction coating on such surfaces. Thermal spraying, whether by wire arc, plasma or flame, will project the particles in a relatively wide spray pattern at very high velocities from a gun nozzle that is stationed relatively close to the bore surface due to the restricted diameter of conventional cylinder bores (typically 4-6 inches). The combination of such high velocity and short travel distance will allow a small proportion of the particles to bounce or be deflected from the target surface resulting in over spray. Such over spray contaminates adjacent block surfaces not intended to be coated or such over spray merely fails free of the block as waste material. The adjacent surfaces may comprise the crankcase chamber and support surfaces for the crankshaft.
Early masking techniques used resilient or ablatable plugs to protect holes of components that are being thermally sprayed or painted. The plugs needed to be essentially the same size as the opening and needed to penetrate into the opening so that the resiliency of the plug obtained closure and conformance to the hole. Such plugs are not good for components having restricted access to the openings and are a detriment when the interior of the openings must not be penetrated or obscured to allow for proper spraying. Fixed templates of elastomor material have also been used to cover broad planar surfaces to be protected during spraying or painting. These templates are difficult to use when the component has a complex structure preventing easy access. Gases have also been used as a masking medium; gases are blown across the interior side of a folded or curled sheet material to mask such side from a molten bath of metal into which the folded sheet is dipped. If such technique were to be used with thermal spraying of internal cavities, such gases would interfere with the thermal spray deposition.